The invention relates generally to controlling the yaw attitude of an aircraft and more particularly to a rudder boost system for a manually controlled aircraft which provides to the pilot a variable power assist on the rudder pedals which is proportional to the force level which the pilot is manually applying to the controls. Under most conditions of flight, rudder control forces are required for various flying operations, such as take-off climb, coordinated turn crosswind landings and taxing of the aircraft once on the ground. With a manually controlled aircraft, many of these rudder use conditions require minimal rudder pedal forces which the pilot can easily handle. However, under certain situations such as engine failure on take-off, a substantial rudder force is required, due to asymmetrical engine thrust, to maintain the aircraft on a correct flight path without side-slip and roll. A rudder boost system provides additional rudder pedal force to augment the pilot's manual control of the rudders. The rudder boost system of the present invention engages only after the manual force applied to the rudders has exceeded a certain level and the flaps of the aircraft are partially extended. As additional manual force is applied to the rudder pedals, the amount of power boost is proportionally increased from zero to a maximum boost level.